This invention relates generally to solid-state photodetectors wherein photogenerated minority carriers are collected in storage potential wells. More specifically, the invention relates to methods, circuits, and apparatus for obtaining nonlinear responses from arrays of such photodetectors.
A metal-oxide-semiconductor (MOS) sandwich structure wherein the metal electrode is insulated from the semiconductor substrate by a thin oxide layer can function as a light detector under appropriate circumstances. If a voltage of appropriate polarity is applied across the MOS device, the majority carriers are removed from the semiconductor region that is closest to the metal electrode and a potential well is formed that is capable of storing minority carriers. Photons that pass through the thin metal electrode and into the semiconductor are absorbed, thereby freeing minority carriers that are collected in the potential well. The number of minority carriers that are collected in the potential well is a measure of the number of photons that have entered the semiconductor. Equivalently, the number of collected minority carriers is a linear measure of the intensity of the light to which the device has been exposed. The term MOS, as used herein, is intended to include structures wherein another conducting material is substituted for the metal and another insulator is substituted for the oxide.
The MOS photodetector is used to measure the intensity of incident light by (1) dumping any charge that is present in the well, (2) exposing the photodetector to the light for an interval of time, and (3) measuring the charge collected in the well at the completion of the exposure. The minimum light intensity I.sub.min that can be measured is limited by thermally-generated minority carriers that collect in the well during the exposure. The maximum light intensity I.sub.max that can be measured is usually limited by the quantity of charge that can be stored in the potential well. The resulting dynamic range of an MOS photodetector is equal to I.sub.max /I.sub.min.
For many applications it is desirable that the charge collected in the potential well of an MOS photodetector be a nonlinear monotonically-increasing function of light intensity in order to increase I.sub.max and the dynamic range of the device. This approach can also provide improved measurement precision at low light intensities.
A method of achieving nonlinear functional relationships between collected charge and light intensity is described by Sayag in U.S. Pat. No. 5,055,667 issued Oct. 8, 1991 which is incorporated by reference. The essence of the method is to bleed off charge from the potential well as charge collects in the well during exposure to light. The amount of charge bled off during the exposure is made to depend on the light intensity--the amount being increasingly larger as the intensity becomes larger. The practice of Sayag's invention requires that the photodetectors have special sink regions to receive the excess charge from the photodetector sites.
The present invention provides a means for practicing the method described by Savag and also includes a method of charge collection control not disclosed by Sayag.